In many applications, insulated conductive wires are exposed to constant bending forces. One example occurs with an implanted pacemaker, where the pacemaker electrodes are bending with each heart beat. Another common example occurs in any type of machine having two relatively moving parts connected by a conductive wire.
In applications of conductive wires such as these, wire fatigue or insulator fatigue may become a serious limitation to the lifetime of the integrity of the conductor. Fatigue may be a problem particularly where the insulative material is subject to repeated stress fatigue and/or it is impractical to check and replace wires. This is the problem currently encountered with pacemaker leads, where the nature of polymer is limited by the need for biocompatibility and there is considerable expense and medical risk in replacing the leads.
The invention includes, in one aspect, an insulated, fatigue-resistant conductor article having as its elements, a conductive wire, a polymeric insulative sleeve having inner and outer layers, and a shape memory alloy (SMA) element having a thickness between 2 and 250 microns, preferably 2-100, more preferably 2-50 microns, an undeformed austentitic state, an Af between about xe2x88x9210xc2x0 C. and 35 C., a pseudoelasticity character above its Af, and demonstrating a stress/strain recovery greater than 3% above its Af.
The wire is encased in an inner layer of the sleeve, the inner layer of the sleeve is surrounded by the SMA element, and the SMA element is encased in the outer layer of the sleeve. The SMA element can undergo pseudoelastic expansion by stress-induced martensite in response to bending of the conductor article, to resist bending fatigue and thereby prevent the polymeric insulative sleeve from cracking or splitting in response to fatigue in the sleeve material.
The SMA element may have a selected a selected curvature along its length in its austentite form, biasing the article toward this curvature in the absence of a bending force applied to the wire. Alternatively, the SMA element may be substantially straight along its length in its austentite form, biasing the article toward a straight condition in the absence of a bending force applied to the wire.
In various embodiments, the SMA element is (i) a thin-film ribbon helically wound about the inner-sleeve layer, wherein the ribbon has a thickness of between about 2 and 100 microns, a ribbon width between about 0.5-20 mm, and where the ribbon may have a variable pitch along its length, producing a SMA material gradient along the length of the article; (ii) a thin-film cylindrical sleeve having a thickness preferably of between about 2 and 50 microns; (ii) an SMA wire or ribbon braid, (iv) a coiled SMA wire; or (v) a plurality of elongate SMA wires or ribbons, each extending substantially along the length of the article between the two sleeve layers.
The inner and outer insulative sleeves may have the same or have different polymer compositions; where the article is a pacemaker lead or other body-implantable wire, the outer sleeve layer is formed of a biocompatible polymer.
In another aspect, the invention includes a pacemaker having, as pacemaker leads, conductive articles in accordance with the article above.
In still another aspect, the invention includes a method of forming the conductive article above. The method uses the elements of: an elongate conductive wire, a polymeric material, and an elongate thin-film shape memory alloy (SMA) element having a thickness between 2 and 250 microns, an undeformed austentitic state, an Af between about xe2x88x9210xc2x0 C. and 35 C., a pseudoelasticity character above its Af, and demonstrating a stress/strain recovery greater than 3% above its Af. These elements are combined by coextrusion to form the wire article. The article formed by coextrusion may lack the outer polymer sleeve, in which case the article is further treated to coat the article with an outer polymer coating, e.g., a biocompatible polymer coating.